Trading resumed on the New York Stock Exchange floor around 3:10 p.m. ET Wednesday after a technical issue caused a more than three and a half hour halt.
Trading stopped around 11:30 a.m. ET due to what the exchange called an "internal technical issue." The NYSE said it did not suspect a cyberattack.
"I can't say with precision exactly what drove it," NYSE President Thomas Farley told CNBC. "We found what was wrong and we fixed what was wrong and we have no evidence whatsoever to suspect that it was external."
"There is no indication that there are malicious actors involved," White House Press Secretary Josh Earnest said, adding that the White House is monitoring the situation.
The issues did not affect the electronic NYSE Arca and NYSE Amex/Arca Options. The Nasdaq reported no technical problems, saying it continued to trade NYSE-listed stocks during the halt.
By about 3:15 p.m. ET, the NYSE said "all systems are functioning normally" except the Openbook data feed for the NYSE MKT primary markets. Volume was light after floor trading restarted and closing auctions continued as normal.
The NYSE said all open orders before the halt, including market on close, limit on close and closing offset, were canceled.
"Do we need to change our protocols? Absolutely. This can't happen again. We can't put ourselves in this position again. Exactly what those changes are, I'm not yet prepared to say," Farley said.
Just after 8 a.m. ET on Wednesday morning, the NYSE put out an alert about "a reported issue with a gateway connection" affecting certain symbols. It issued a notice at 10:37 a.m. ET that the problem was resolved.
"We opened the market as usual. All the NYSE-listed stocks opened without incident. Around mid-morning, we started to see some concerns about the way trading was occurring. Customers weren't getting all of the messages back that you otherwise expect," Farley said.
NYSE floor trading volume was 444 million on Wednesday, about half of the 30-day average of 861 million. Still, trading on the NYSE floor makes up only about a quarter of consolidated tape volume, or trading of all NYSE securities on all platforms, based on a recent 30-day average.
"This will not cause a move in any particular direction, so I would kind of wait it out and see what happens," said Art Cashin, UBS director of floor operations at the NYSE, in a CNBC interview.
Retail investors were largely unaffected by the outage, said JJ Kinahan, chief strategist at TD Ameritrade.
"This is going to make it more difficult to say the floor traders are important when there's a technical problem. Because the technical problems take out the floor traders, too," said Richard Repetto, an analyst with Sandler O'Neill & Partners.
Officials widely denied any link to a cyberattack.
The FBI said in a statement it would "monitor the situation" but "no further law enforcement action is needed at this time." Securities and Exchange Commission Chair Mary Jo White in a statement said "we are in contact with the NYSE and are closely monitoring the situation."
Homeland Security Secretary Jeh Johnson noted the glitch that caused the trading halt and an earlier computer problem that grounded United Airlines flights appeared unrelated.
On Tuesday, hacker group Anonymous tweeted a suspicious statement about potential problems on Wall Street.
Separately on Wednesday, The Wall Street Journal website experienced an apparent outage that was resolved by roughly 1 p.m. ET.
This story is developing. Please check back for further updates.
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